WASHINGTON (CNN) --The United States has evidence that Iran has secretly been building large nuclear facilities -- sites that could possibly be used to make nuclear weapons, senior U.S. officials tell CNN.

Commercial satellite photographs taken in September show a nuclear facility near the town of Natanz and another one near Arak, the officials said. (View map)

But Iran's foreign ministry spokesman said the country's only nuclear activity is of a peaceful nature, and its facilities have been "regularly and frequently" inspected by the International Atomic Energy Agency, or IAEA.

"Iran hasn't committed any acts that can be considered against international rules, and will not do so in the future," Hamid Reza Assefi told CNN. "At the same time, no country could, for its own political objectives, prevent Iran from achieving its own goals."

A spokesman at the IAEA in Vienna, Austria confirms the agency is seeking access to the two sites and has so far been put off by Iran.

The vice chairman of the U.S. Senate Intelligence Committee said the development was "disturbing news."

"We don't need another nuclear power -- not with Iran sponsoring terrorism that it has in the past," said Sen. Richard Shelby, an Alabama Republican. "The fact that they are seemingly pursuing an avenue to build nuclear weapons should be disturbing to everybody."

Assefi said the United States was trying to start a negative publicity campaign to divert attention from other issues.

"This kind of publicity is not new," Assefi said. "Certain circles within the United States are trying to create tensions and poison the international atmosphere, and to avert international public opinions away from the real regional danger, which is Israel."

Iranian dissidents have long contended that Iran has been working on nuclear capabilities. But the new satellite photographs and the conclusions drawn from them by nuclear experts are the first evidence to support such claims.

Heavy Water (D2O)

Water in which both hydrogen atoms have been replaced with deuterium, an isotope of hydrogen

Heavy-water-moderated reactors can be used to make tritium, an ingredient of thermonuclear weapons

Source: Federation of American Scientists

Nuclear expert David Albright said the size and secrecy of the program suggest Iran might be working toward building nuclear weapons.

Albright is head of the Institute for Science and International Security (ISIS), which identified the photographs. The non-profit, non-partisan ISIS focuses on stopping the spread of nuclear weapons.

The satellite picture of the facility near Arak concerns nuclear experts.

Corey Hinderstein, also of ISIS, said the site resembles heavy water plants found in Pakistan and contains a similar Z-shaped structure.

The large facility at Natanz appears to U.S. intelligence officials to be a uranium-enrichment plant, and civilian experts, including Hinderstein, agree.

Iran has a declared nuclear program at Bushehr that is designed to produce nuclear power for electricity only, according to the country's U.N. ambassador.

Commercial satellite photo of a nuclear facility near Natanz, Iran

"I can categorically tell you that Iran does not have a nuclear weapons program," Mohammed Javad Zarif said in an interview with CNN. "Any facility we have ... if it is dealing with nuclear technology, it is within the purview of our peaceful nuclear program." (Transcript of interview)

Iranian officials say a visit by senior IAEA officials is expected in February. IAEA officials say they want to visit Arak and Natanz on that trip.

IAEA officials also point out that nothing Iran is known to have done has violated international law.

Bush labeled Iran part of an "axis of evil," along with Iraq and North Korea, in his State of the Union address this year.